1. Field of the Invention
This invention relates to an optical information recording medium adapted to be irradiated with a laser beam in order to record information by means of a rewritable optical information recording/reproduction apparatus and to a method of manufacturing the recording medium. More particularly, it relates to a phase change type optical disk medium adapted to record information by bringing the phase state of its recording layer into either an amorphous state or a crystalline state and reproduce information by utilizing the optical characteristics of the recording layer that differ between the phase states and to a method of manufacturing the disk medium.
2. Description of Related Art
Optical information recording/reproduction employing a laser beam has been and is utilized for large capacity memories in various fields because the optical head for optical information recording/reproduction can be made as an access to a recording medium very fast and in non-contact therewith, and record a large volume of information to or reproduce a large volume of information from the medium. Optical information recording mediums that can be used for optical information recording/reproduction are classified into the reproduction-only type such as compact disks and laser disks from which the user can only reproduce data, the write-once type on which the user can record additional data and the rewritable type on which the user can record and from which the user can erase data repeatedly for rewriting. The write-once type and trewritable type optical information recording mediums are being used as external memories of computers and mediums for storing document files and image files.
The rewritable optical information recording mediums include phase change type optical disks that utilize phase changes of a recording film and magneto-optical disks that utilize changes of the direction of magnetization of a vertically magnetized film. Of these, the phase change type optical disks are expected to be in the main stream of rewritable type optical information recording mediums because, unlike magneto-optical disks, information can be recorded on them without requiring an external magnetic field and overwritten with ease.
In conventional phase change type optical disks, the value selected for the light absorption coefficient Aa of the recording layer in the amorphous state is generally higher than the value selected for the light absorption coefficient Ac of the layer in the crystalline state. Therefore, as the pitch of arrangement of recording tracks of a phase change type optical disk medium is made narrower in order to increase the recording density, the recording marks arranged at adjacently located recording tracks where information is already recorded and that are in the amorphous state, showing a high light absorption coefficient, absorb the laser beam at a high rate. Then, as a result, the temperature of the recording marks in the amorphous state rises to crystallize the recording marks and consequently erase the information recorded on the recording marks. In other words, cross erasures take place there.
This problem can be effectively prevented from occurring by setting a value for the light absorption coefficient Aa of the recording layer in the amorphous state lower than the value for the light absorption coefficient Ac of the layer in the crystalline state. A technique of making Aa lower than Ac has already been proposed. According to this technique, a first dielectric layer, a second dielectric layer, a third dielectric layer, a first interface layer, a recording layer, a second interface layer, a fourth dielectric layer and a reflection layer are sequentially laid and the refractive index n2 of the second dielectric layer and the refractive index n3 of the third dielectric layer are made to show a relationship of n2<n3, while the refractive index n1 of the first dielectric layer and the refractive index n2 of the second dielectric layer are made to show a relationship of n2<n1. More specifically, ZnS—SiO2 films showing a refractive index of about 2.3 are used for the first and third dielectric layers of the above dielectric layers, and an SiO2 film showing a refractive index of about 1.5 or an Al2O3 film showing a refractive index of about 1.7 is used for the second dielectric layer as general practice. Alternatively, an SiN film showing a refractive index of about 1.9 may be used for the second dielectric layer depending on the situation where the recording medium is used (see, inter alia, JP-A-2000-90491 and 2000-105946).
When SiO2 film or Al2O3 film is used for the second dielectric layer, generally an SiO2 target or an Al2O3 target is used for the film forming process. However, it has been pointed out that the film forming rate of the process is low and hence such a process is not suitable for mass production. When, on the other hand, an SiN film showing a refractive index of about 1.9 is used for the second dielectric layer, the above described fourth dielectric layer inevitably needs to be made relatively thick in order to meet the requirement of Aa<Ac. Thus, it has also been pointed out that the repeat O/W resistance of the recording medium can be degraded.
In an attempt of solving the above-identified problems, a technique of forming an SiON film in a mixed gas atmosphere of argon gas, oxygen gas and nitrogen gas, using an Si target, has been proposed to replace the above described SiO2, Al2O3 or SiN film. It has been reported that a higher film forming rate can be achieved and dielectric film showing a relatively low refractive index can be produced when such SiON film is used in place of SiO2, Al2O3 or SiN film. The use of such a film is advantageous in terms of mass production and repeat O/W resistance (see, inter alia, Proceedings of the 15th Symposium on Phase Change Optical Information Storage PC0S2003, pp. 56-61 (2003)).
However, the above-described known technique is accompanied by the following problems. As pointed out above, the SiON film that is formed in the mixed gas atmosphere of argon gas, oxygen gas and nitrogen gas, using the Si target, provides a film forming rate about three times as high as the film forming rate of the conventional process for producing an SiO2 or Al2O3 film but considerably lower than the film forming rate of the process of forming ZnS—SiO2 film, which is popularly used for phase change recording mediums. More specifically, the film forming rate of forming the SiON film is about two-thirds that of forming the ZnS—SiO2 film. Thus, the problem remains unsolved that the productivity is not satisfactory due to the unbalanced film forming rates when the SiON film acting as the dielectric layer is sandwiched between upper and lower ZnS—SiO2 films.
On the other hand, in the case of recording mediums realized by using SiON film that is formed in a mixed gas atmosphere of argon gas, oxygen gas and nitrogen gas, using an Si target, it has been known that the reflectivity of the medium may change before and after an environment test and falls after the environment test if the medium is held in a high temperature/high humidity condition. Generally, it is desired that the reflectivity of the recording medium does not change before and after the environment test. In other words, the instability of reflectivity may become a problem when the recording medium is used for a long period of time.